The conventional practice when drilling oil and gas wells is to use a drilling fluid or “mud” to provide certain engineering functions and to enhance the efficiency of the drilling operation. Typically, a drilling fluid is continuously circulated from a storage area on the surface, down through the drill pipe, and then returned through the borehole to the surface. One function of the drilling fluid is to act as a carrier to remove drilled solids, such as cuttings and formation carvings, from the borehole to the surface. The transfer of cuttings to the surface is effective because the fluids posses a certain rheology to suspend and carry the solids, and various viscosifying components are usually included in the drilling fluid for this purpose.
The fluid can also function to cool and lubricate the drill bit, to prevent pressure from being lost if a void or gas pocket is encountered, and to reduce the introduction of fluids, salts, and solids into the geological formation encountered in the operation. This latter function becomes extremely important when drilling through formations of clay or shale. Shales are composed of primary clay minerals, which may be altered when contacted with water. For example, the smectites are notorious for their considerable swelling and expansion when contacted with water, due to water penetrating the interlayer spaces of the clay. As a result, when clay and shale are encountered in a drilling operation, their contact with water can induce swelling, which may cause deformation of the wellbore wall, reduced well diameter, and binding of the drill pipe or bit in the formation. Brittle fractures of the formation also can develop, and the fractured clay chips will precipitate to the bottom of the well adjacent the drill bit, possibly blocking its retrieval and movement of the drillstring. Therefore, one of the most desirable functions of a drilling fluid is to limit or minimize the effects of water on shales and clays that are encountered in a drilling operation.
As a result, there is a continuing need for new drilling fluid compositions and methods that are effective in reducing or minimizing the fracturing, swelling, or dispersion of shales and clays when the fluid contacts these formations. It would be desirable if effective drilling fluid compositions could be developed that are more environmentally benign than those currently in use, to meet more stringent industrial standards. This environmental goal has assumed a new sense of urgency as, for example, regulatory pressures such as the European Union's (EU's) REACH (Registration, Evaluation, Authorization and Restriction of Chemicals, 2006) Regulations seek to improve protection of human health and the environment from chemical risks. It would also be desirable if new completion fluids, work-over fluids, and stimulation/fracking fluids could be developed that are similarly environmentally sound.